Optical parametric oscillators are well-known, non-linear optical devices capable of producing coherent radiation at a desired frequency via parametric amplification. In a conventional optical parametric oscillator (OPO), a pump source supplies a beam of laser light at a pump wavelength to an optical cavity bounded by end mirrors and containing a non-linear optical medium such as a non-linear optical crystal. As the pump beam propagates through the non-linear crystal within the cavity, photons at the pump wavelength are converted into photon pairs at two longer wavelengths, resulting in two lower-energy beams at these two wavelengths, conventionally called the signal wavelength and the idler wavelength. The sum of the frequencies of the signal and idler beams equals the frequency of the pump beam. The particular wavelengths of the signal and idler beams are determined by a number of factors, including: the pump wavelength, the type and cut of the non-linear crystal, and the design of the optical cavity. In addition, by adjusting the angle of the non-linear crystal, tuning of the signal wavelength and idler wavelength beams can be achieved.
Since typical operating conditions cause only a small fraction of the pump beam to be converted to the signal and idler beams in the initial pass through the non-linear optical crystal, the optical cavity of the OPO is generally designed to oscillate one or both of the parametrically generated beams such that the signal and/or idler beam is amplified in successive passes through the non-linear optical crystal. The OPO is considered a doubly resonant oscillator when both of the generated optical beams are resonated and is considered a singly resonant oscillator when only one of the generated optical beams is resonated. Specifically, the optical cavity can be designed with end mirrors which reflect only one of the signal and idler frequencies (singly resonant) or with end mirrors which reflect both the signal and idler frequencies (doubly resonant).
One application of an OPO is a light source for detection and/or identification of chemical/biological gases. Chemical or biological agents with narrow linewidths require laser linewidths a small fraction of their absorption features for accurate measurements, while other agents with broad features require multiple laser lines and linewidths consistent with atmospheric transmission windows. Thus, an OPO for such applications needs to have a tuning range of multiple (10 or more) wavelengths, be continuously tunable over this range and be capable of operating at a high repetition rate to allow for signal averaging measurements. Moreover, it is desirable to provide and OPO with a high frequency tuning rate for short collection times of data and quick assessment of a sampled region. Finally, the OPO should be relatively light and compact so that it can be deployed on a variety of platforms, including air and land vehicles.